The saw-mills of the present day are of two distinct kinds ; the circular, those that cut by a continuous rotatory motion, and the reciprocating, which operate as the common pit or frame-saw. The circular sawmills are for the most part used for cutting up timber of small dimensions ; and the reciprocating for large timber, in forming beams, rafters, planks, &c. out of large timber. The most important machinery of the kind was erected by Mr. Brunel, at Portsmouth, to whom the mechanical world is indebted for many important inventions and improvements.
- Having thus briefly sketched the history of this important theist') simple invention, we shall proceed to the description of its variously modified forms, and the processes employed at Sheffield in their manufacture ; to which we shall add an account of the general arrangement of saw-mills, and a more detailed explanation of some improvements, by which its utility has been extended.
Saws are made of a great variety of forms and sixes, to adapt them to the materials on which they are designed to operate. The most common are those used by carpenters, who require in ordinary no lees than ten different saws; namely, a crosscut saw, for dividing a tree or log transversely, by means of two workmen, one on each side, who alternately pull the saw towards them, the teeth being made to cut equally in each direction ; a pit-saw, for sawing the logs up into planks or scantlings, the operation being performed in a pit by a vertical motion of the saw, and usually by a class of workmen called sawyers ; a large frame-saw, which is a saw-plate five, six, or seven feet long, stretched in a frame, and used to cut timber longitudinally with greater nicety than the pit-sase ; a sipping-saw, which is a hand-saw, with a blade twenty-eight or thirty inches long, an having large teeth for ripping, or cutting out stuff coarsely and quickly ; a hand i -saw (peculiarly so denominated), usually provided with a twenty-six inch blade, and angular teeth, five to the inch ; a panel-saw is the same as the but with finer teeth, (seven or eight to the inch,) for cutting stuff very clean, and for the more delicate or exact species of work. Saws with very fine teeth, and very thin blades, stiffened with stout pieces of iron or brass, rivetted to the back edge, are also used, of several kinds, which are distinguished by the several terms, cassia, sash, carcase, and tenon, indicative of their uses, and also of their sizes, which vary from six to twenty inches in length ; several very narrow saws, indifferently called lock, compass, key-hole, and turning saws, for cutting out small pieces, and rounding work : small frame-saws, six or eight inches long, are sometimes required by the carpenter for cutting bah wood and iron ; the teeth of the latter being smaller and mere obtuse than the former.
There are many saws used by other mechanics which differ from the carpenters, the details of which would be uninteresting ; we shall therefore proceed to take a brief notice of the process of manufacturing saws, as practised at Sheffield, from whence three-fourths of the inhabitants of the globe are supplied.
• The very commonest kind of saws are made of iron plates, hammer hardened, and planished upon an anvil, to give them some degree of stiffiress and elasticity. Such instruments are, of course, spurned by the workmen ; nevertheless, as their cost is but trifling, they are purchased in great quantities by those who consider any saw to be better than no saw at all.
Thu more useful saws which workmen employ are made, nominally, of either shear or cast steel; but the quality of these materials may differ, as well as. the Dews made of them, in every possible degree. The common test of. a good saw, that of bending it into a bow,. and letting it spring again into.* straight line, is considered by some persona as a fallacious and unnecessary test, and that it sometimes spoils a saw, possessing in other respects all the properties of a valuable tool. A dispute has been raised on this point, and ably advocated on both sides. For our own parts, we would simply. observe, that such process of springing infallibly proves two of the essential properties of a good saw, namely, uniformity of thickness in the blade, and perfect elasticity ; properties indispen sably necessary to the unskilful, who have not acquired the tact of holding the saw lightly and moving itconstantly in the same plane ; and who, in consequence fre9uently bend the saw, which is thus infallibly spoiled ; it'does not spring bacli touts previous straightness.